Flash memory cards have gained wide acceptance for its non-volatile flash-memory storage, which is ideal for portable devices that may lose power, since the data is not lost when stored in the flash memory. Flash memories are constructed from electrically-erasable programmable read-only memory (EEPROM) cells. Since flash memory is solid-state, moving parts are not present. Flash does not fail under moderate shock or vibration that would cause a failure in a rotating disk.
Rather than use a randomly-addressable scheme such as is common with dynamic-random-access memory (DRAM), many flash memories use a block-based addressing where a command and an address are sent over the data bus and then a block of data is read or written. Since the data bus is also used to send commands and addresses, fewer pins are needed on the flash-memory chip, reducing cost. Thus flash memory is often used as a mass-storage device rather than a randomly-addressable device.
A popular bus standard is Multi-Media Card (MMC). An extension of MMC is known as Secure Digital (SD). MMC and SD flash devices are common today.
As process technologies improve and devices are scaled down in size, the power-supply voltage needed to operate integrated circuits decreases. Some devices such as flash memory chips may operate at 3.3-volt power supplies, while other devices such as controller chips may operate using a 1.8-volt power supply.
More recently, bus standards have been extended to operate at lower voltages. For example, the SD bus may operate using a 3.3-volt power supply or the SD bus may operate at a reduced 1.8-volt power supply. Larger buses may include several power-supply pins for the different power-supply voltages. However, since the SD bus has a limited number of pins, the SD bus has only one power supply pin, which carries either 1.8 or 3.3 volts, but not both. Thus SD devices receive only one power-supply voltage.
An older SD card that needs 3.3 volts to operate may be plugged into a newer host that drives 1.8 volts onto the power-supply pin. This older SD device cannot operate properly with the newer host. A newer SD card may require 1.8 volts and may not operate properly when inserted into an older host that supplies 3.3 volts. Thus voltage compatibility issues arise.
What is desired is a dual-voltage SD card. A dual-voltage SD card and controller that can receive either 3.3 or 1.8 volts from the power-supply pin of the SD bus is desirable. An SD controller chip using an advanced process that can operate at 1.8 volts is desired, even when the SD card is inserted into a 3.3-volt SD host.